Voltage Drop Calculator

Solve any of the three questions every conductor run raises: how much voltage a circuit drops, the minimum conductor size that keeps the drop within limits, or the maximum distance a run can cover before it falls out of compliance. Supports NEC AWG/kcmil conductors (14 AWG through 1000 kcmil, characteristics based on NEC Chapter 9 Tables 8 and 9) and international IEC metric cables (1.5–630 mm²), copper or aluminum, DC, single-phase, and three-phase AC, with selectable 60/75/90°C conductor temperature, PVC/aluminum or steel conduit, parallel conductor sets, and load entry in amps, kW, kVA, or horsepower with power factor. Results include the voltage drop, percent drop, voltage at the load, a color-coded verdict against the NEC 3% branch-circuit and 5% total recommendations, and a wire-size comparison chart that shows at a glance whether the next conductor size brings the run into compliance. Built for electricians, electrical engineers, solar and EV installers, and students.

Pick a calculation mode, set the circuit parameters, and click Calculate. Core formula: VD = k × I × L × Z ÷ 1000 ÷ N (k = 2 for DC/single-phase, √3 for three-phase).

Standard

Phase

Conductor Temp

Conduit Type

Conductor Material

Wire Size (AWG / kcmil)

Supply Voltage (select or enter custom)

Load (value + unit)

Power Factor (AC — used for kW / hp conversion and impedance)

One-Way Circuit Length

Parallel Conductor Sets (1 = standard single run)

Max. Allowable Voltage Drop (% — NEC recommends 3% branch)

⚠️ Motor-load calculation (horsepower input): motor starting current, efficiency, service factor, and NEC Article 430 requirements are not modeled by this estimate. Have a licensed electrician verify conductor sizing for any motor circuit.

Results

Voltage Drop

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Percent Drop

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Voltage at Load

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Load Current

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Formula: —

Wire-Size Comparison

Each bar is the percent drop for a neighboring wire size on the circuit you entered. The size in use is marked in gold. The dashed line is the 3 percent branch-circuit mark from NEC 210.19 and 215.2. One size either way is often what decides whether the run passes.

Neighboring Sizes Your Size 3% NEC Recommendation

🧰 Sizing conduit for these conductors? Voltage drop and conduit fill go hand in hand on every run — use our Conduit Fill Calculator to check how many conductors your raceway can legally carry.

Results are engineering estimates based on published conductor characteristics at the selected operating temperature. Actual voltage drop varies with installation conditions. Verify with a licensed electrician before any electrical work.

How to Use the Voltage Drop Calculator

Choose the Calculation Mode

Pick Voltage Drop to check an existing or planned circuit, Min. Wire Size to have the calculator select the smallest compliant conductor, or Max. Distance to find how far a run can go before exceeding your voltage-drop limit.

Set the Circuit Parameters

Select the wiring standard (NEC AWG/kcmil or IEC mm²), the phase (DC, single-phase, or three-phase AC), the conductor material, the 60/75/90°C temperature rating, and — for AC circuits — the conduit type.

Enter the Load and Run Details

Enter the supply voltage (or pick a common preset), the load in amps, kW, kVA, or horsepower (with power factor where applicable), the one-way circuit length in feet or meters, and the number of parallel conductor sets.

Calculate, Review, and Print

Click Calculate to see the voltage drop, percent drop, voltage at the load, the color-coded NEC 3%/5% verdict, and the wire-size comparison chart. Use Print to capture results and the chart exactly as they appear on screen.

Guide to Voltage Drop Comprehensive Guide

What voltage drop is, why the NEC cares about it, and what every selector on this calculator actually changes in the math.

What Voltage Drop Is

Every conductor has resistance, so some of the supply voltage is consumed pushing current through the wire itself instead of powering the load. The longer the run, the smaller the conductor, and the higher the current, the more voltage is lost. The symptoms of excessive drop are familiar on any job site: dim or flickering lights at the end of a long run, heaters that underperform, and motors that run hot and fail early because low voltage forces them to draw more current.

The NEC 3% / 5% Recommendations

Informational Notes to NEC 210.19 and 215.2 recommend limiting voltage drop to 3% on a branch circuit and 5% combined across the feeder and branch circuit. These are design recommendations rather than enforceable requirements in most jurisdictions — but they are the targets inspectors, engineers, and manufacturers expect, and some applications (such as sensitive electronics and fire pumps) carry stricter requirements. This calculator's verdict banner grades your circuit against both thresholds.

60 / 75 / 90°C Conductor Temperature

Conductor resistance rises with temperature — roughly 0.3% per °C for copper and aluminum — so the same wire drops more voltage when running hot. The three selectable ratings correspond to the standard NEC insulation temperature columns. 75°C is the everyday industry assumption; 60°C models a lightly loaded conductor; 90°C models a fully loaded conductor with 90°C-rated insulation. Note that NEC 110.14(C) usually limits terminations to the 60°C or 75°C rating regardless of the wire's insulation, which is one of several reasons results at any setting are estimates that a licensed electrician must verify.

PVC / Aluminum vs. Steel Conduit (AC only)

Alternating current in a conductor creates a changing magnetic field, and the raceway around it affects the resulting reactance. Steel conduit is magnetic and raises the effective reactance compared with PVC or aluminum raceway — NEC Chapter 9 Table 9 publishes separate columns for the two cases. On small branch-circuit conductors the difference is minor because resistance dominates; on large feeders, reactance is a substantial share of total impedance and the conduit selection visibly changes the result. DC circuits have no reactance, so the selector hides in DC mode.

NEC (AWG / kcmil) vs. IEC (mm²) Mode

NEC mode covers the North American size ladder from 14 AWG through 1000 kcmil using resistance and reactance characteristics based on NEC Chapter 9 Tables 8 and 9. IEC mode covers the international metric ladder from 1.5 mm² to 630 mm², with resistance computed from standard IEC 60228 copper and aluminum resistivity and corrected to your selected operating temperature. Switching standards also switches the default length unit between feet and meters — either unit works in either mode.

Loads in kW, kVA, and Horsepower

When you enter a load in kW or hp, the calculator first converts it to current using the supply voltage and your power factor (I = P ÷ (V × PF) single-phase, with a √3 factor for three-phase). kVA entries divide by voltage alone, since kVA already represents apparent power. For horsepower, 1 hp = 746 W is applied and the power-factor field effectively carries the combined power factor and efficiency of the motor — a simplification, which is why motor circuits get an additional verification warning and should always be checked against NEC Article 430 by a licensed electrician.

Parallel Conductor Sets

Large feeders often run multiple conductors per phase in parallel. Two identical parallel sets halve the effective impedance, cutting voltage drop in half; three sets cut it to a third. The NEC permits parallel conductors 1/0 AWG and larger (with matching length, material, and size per NEC 310.10), so treat parallel results for small conductor sizes as theoretical.

This guide is provided for educational and reference purposes. Always confirm circuit design against the current edition of the National Electrical Code (NFPA 70), local amendments, and the authority having jurisdiction before any electrical work.

Worked Examples

Three voltage-drop jobs, worked from a load, a length, and a wire size. Load an example to run that job in the calculator above.

1. Detached Garage Feeder — 240 V, 50 A, 150 ft

A 240 V single-phase, 50 A subpanel feeder runs 150 ft to a detached garage in 6 AWG copper, PVC conduit, 75°C. Does it meet the NEC 3% recommendation?

VD = 2 × 50 A × 150 ft × Z(6 AWG) ÷ 1000 ≈ 7.4 V → ≈ 3.1% — marginal; 4 AWG drops it to ≈ 2.0%

2. Well Pump Motor — 230 V, 2 hp, 300 ft — Min. Wire Size

A 2 hp, 230 V single-phase well pump sits 300 ft from the panel, power factor taken as 0.85. What is the smallest copper conductor that keeps the drop within 3%?

I = 2 × 746 ÷ (230 × 0.85) ≈ 7.6 A → Min. Wire Size mode selects the first size with VD ≤ 3%

3. IEC Metric Run — 230 V, 4 kW, 40 m — Max. Distance Check

A 230 V single-phase 4 kW load (PF 0.9) is wired in 2.5 mm² copper. How far can the run go before exceeding a 3% drop?

I = 4000 ÷ (230 × 0.9) ≈ 19.3 A → Max. Distance mode solves L for VD = 3% of 230 V

Quick Reference Table

Typical copper circuits at 75°C in PVC/aluminum conduit, computed with this calculator's NEC-based method — for quick lookup without re-entering numbers.

Circuit	Wire (Cu)	Approx. Drop	Within NEC 3%?
120 V · 15 A · 50 ft · 1Ø	14 AWG	≈ 3.9%	No — use 12 AWG (≈ 2.5%)
120 V · 20 A · 50 ft · 1Ø	12 AWG	≈ 3.3%	Borderline — 10 AWG brings it to ≈ 2%
240 V · 30 A · 100 ft · 1Ø	10 AWG	≈ 3.0%	At the limit — 8 AWG recommended
240 V · 50 A · 150 ft · 1Ø	6 AWG	≈ 3.1%	Borderline — 4 AWG brings it to ≈ 2%
208 V · 60 A · 200 ft · 3Ø	4 AWG	≈ 3.1%	Borderline — 3 AWG clears 3%
480 V · 100 A · 300 ft · 3Ø	1 AWG	≈ 1.8%	Yes
12 V DC · 20 A · 25 ft	10 AWG	≈ 10.3%	No — low-voltage DC needs large wire; 4 AWG ≈ 2.6%

Values are rounded estimates for comparison. Run your exact circuit in the calculator above and verify with a licensed electrician.

Frequently Asked Questions

What is an acceptable voltage drop?

The National Electrical Code (NFPA 70) recommends limiting voltage drop to 3% on a branch circuit and 5% combined across the feeder and branch circuit (NEC 210.19 and 215.2 Informational Notes). These are recommendations rather than strict requirements in most cases, but they are the accepted design targets — excessive drop causes dim lighting, overheated motors, and poor equipment performance.

What is the voltage drop formula?

For DC and single-phase circuits: VD = 2 × I × L × Z ÷ 1000 ÷ N. For three-phase circuits: VD = 1.732 × I × L × Z ÷ 1000 ÷ N. I is the load current in amps, L is the one-way length (feet or meters), Z is the conductor impedance in ohms per 1,000 ft (or per km), and N is the number of parallel conductor sets. For AC circuits Z combines resistance and reactance with the load power factor — the exact formula used for your inputs is shown beneath the results.

Does temperature affect voltage drop?

Yes. Conductor resistance rises with temperature, so a conductor operating at 90°C drops more voltage than the same conductor at 60°C. This calculator lets you select a 60, 75, or 90°C operating temperature and adjusts conductor resistance accordingly. Actual operating temperature depends on load, ambient conditions, insulation type, and termination ratings, which is why results must be verified by a licensed electrician.

Copper vs aluminum — which has less voltage drop?

Copper. Aluminum has roughly 60% higher resistance than copper for the same conductor size, so an aluminum conductor must typically be about two AWG sizes larger to match the voltage drop of a copper conductor. Aluminum remains popular for large feeders because it is lighter and less expensive per ampere of capacity — run both materials in the calculator to compare.

Why does conduit type matter for AC voltage drop?

Steel conduit is magnetic, which increases the effective reactance of the conductors inside it compared with PVC or aluminum conduit. On larger AC conductors, where reactance is a significant share of total impedance, this measurably increases the voltage drop. NEC Chapter 9 Table 9 publishes separate values for the two cases, and this calculator applies whichever you select. Reactance does not apply to DC circuits, so the selector hides in DC mode.

What is the difference between NEC AWG and IEC mm² mode?

NEC mode uses North American AWG and kcmil conductor sizes with resistance and reactance characteristics based on NEC Chapter 9 Tables 8 and 9. IEC mode uses international metric cable sizes from 1.5 mm² to 630 mm², with resistance computed from standard IEC 60228 copper and aluminum conductor resistivity. Choose the mode that matches the cable standard used in your region — the default length unit switches between feet and meters to match.